1. Field of the Invention
The present invention relates to an optical information reproducing apparatus (optical disk apparatus), and more particularly, to an optical information reproducing apparatus for recording and reproducing optical information by performing control with a constant linear velocity or a substantially constant linear velocity by varying a rotation frequency of a disk in accordance with a radial-direction position.
2. Description of the Related Art
Conventional rotation control of an optical disk is performed by an optical disk apparatus for a CD (compact disc) or the like that performs rotation control with a constant linear velocity (CLV), or an optical disk apparatus in which the interior of a disk is divided into a plurality of zones and rotation control is performed so as to realize a substantially constant linear velocity (MCLV) between zones.
Recently, optical disk apparatuses have been developed in which a sample servo optical disk having clock marks or wobble marks, the number per track of which is constant, and which are pre-recorded radially from the center of the disk, is rotated using an MCLV process.
FIG. 6 is a block diagram illustrating a conventional optical disk apparatus.
As shown in FIG. 6, the optical disk apparatus includes an optical disk 1, a pickup 2 (having a laser source, a sensor, a focus actuator and a tracking actuator), a detection circuit 3, a focus-error generation circuit 4, a focus-phase compensation circuit 6, a focus-gain circuit 7, a focus-actuator driver 8, a tracking-error generation circuit 9, a tracking-phase compensation circuit 11, a tracking-gain circuit 12, a tracking-actuator driver 13, a spindle 15, a spindle control circuit 16, a spindle driver 17, and a controller 18.
A light beam emitted from the laser source of the pickup 2 is projected onto the optical disk 1. Light reflected from the optical disk 1 is sensed by the sensor of the pickup 2, and is converted into an electric signal by the detection circuit 3.
A focus-error signal is generated from an output from the detection circuit 3 by the focus-error generation circuit 4, and is supplied to the focus-actuator driver 8 via the focus-phase compensation circuit 6 and the focus-gain circuit 7, to control the focus actuator of the pickup 2, thus constituting a focus servo loop. Similarly, a tracking-error signal is generated from the output of the detection circuit 3 by the tracking-error generation circuit 9, and is supplied to the tracking-actuator driver 13 via the tracking-phase compensation circuit 11 and the tracking gain circuit 12, to control the tracking actuator of the pickup 2, thus constituting a tracking servo loop. The spindle driver 17 drives the spindle 15 so as to have a rotation frequency indicated by the controller 18, using the spindle control circuit 16 based on an FG (frequency generator) signal (not shown) or a synchronizing signal from the detection circuit 3, thus constituting a spindle control loop.
In a CLV method, a synchronizing signal from the disk is detected by the detector circuit 3, and rotation control is performed so as to maintain a constant linear velocity based on the synchronizing signal. In an MCLV method, a plurality of zones are provided in a radial direction, and the interior of each zone is controlled so as to have a constant rotation frequency using the spindle control circuit 16 based on the FG signal or the synchronizing signal from the detection circuit 3. In each of the above-described methods, the disk is controlled via CLV or MCLV process so that the rotation frequency is highest at the inner circumference in a radial direction and decreases toward the outer circumference.
In such optical disk apparatuses, a control band for tracking control, focus control and the like is designed so as to be able to perform control within a control error range desired for recording and reproducing operations in accordance with the amount of disturbance provided by disk standards, even if the maximum disturbance permitted by the disk standards is generated. That is, each of the focus-phase compensation circuit 6, the tracking-phase compensation circuit 11, the focus-gain circuit 7 and the tracking gain circuit 12 operates in a fixed control band designed irrespective of the rotation frequency.
FIG. 7 illustrates an example of tracking disturbance in a disk versus the rotation frequency of the disk when the center of the disk deviates from the center of rotation by 100 μm. For example, when the rotation frequency of the disk is 10 Hz, a disturbance of 100 μm is present at 10 Hz. As shown in FIG. 7, the frequency component of the disturbance decreases substantially in inverse proportion to the square of the frequency due to harmonic components or the like. The inclination of the disturbance represents eccentric acceleration of the disk. For example, at 10 Hz, the eccentric acceleration is 0.395 m/s2, and at 20 Hz, the eccentric acceleration is 1.579 m/s2. Accordingly, it can be understood that even if the eccentricity of the disk has a constant value of 100 μm, the frequency component of disturbance becomes larger (so as to move to the right in FIG. 7) a the rotational frequency of the disk becomes higher.
In actual disks, disturbance due to deviation of the center of the disk is the most significant (or the dominant) source of disturbance. The amount of disturbance including harmonic components hardly differs between the inner circumference and the outer circumference.
FIG. 8 illustrates an example of focusing disturbance in a disk.
FIG. 8 represents frequency characteristics of disturbance in a disk. It is assumed that the disk planer and rotates in an inclined state with respect to the plane of rotation, with an inclination of 100 μm at a radius of 50 mm, and the disk is rotated so as to maintain a constant linear velocity of 2.4 m/s at each radius (e.g., relative to a light spot of a recording/reproducing head). With respect to eccentricity, the frequency component of disturbance decreases substantially in inverse proportion to the square of the frequency due to harmonic components or the like. The inclination of the disturbance represents planer inclination acceleration of the disk. For example, the rotation frequency is 10 Hz at a radius of 38.2 mm, and the planer deviation acceleration at that time is 0.302 m/s2. The rotation frequency is 20 Hz at a radius of 19.1 mm, and the planer deviation acceleration at that time is 0.603 m/s2. Although the absolute value of the disturbance component of focus due to inclination with respect to the plane of rotation increases in proportion to the radius, the rotation frequency decreases in inverse proportion to the radius when the disk is rotated with a constant linear velocity. As a result, disturbance decreases as the radius increases (as the rotation frequency decreases).
In consideration of the above-described disturbance characteristics of the disk, in a disk apparatus which maintains a constant (or substantially constant) linear velocity, the standards of the disk must be provided so as not to exceed the standard of disturbance within the range of all rotation frequencies. As a result, the disturbance at the highest rotation frequency serving as the worst-case condition is provided as the standard, and a servo-loop gain is designed so as to suppress the disturbance so as to be maintained within a range of deviation desired for recording and reproducing operations.
However, if the servo loop gain is determined so as to adequately suppress the disturbance at the highest rotation frequency, although no problem arises at the highest rotation frequency, the servo-loop gain has a value more than sufficient at lower rotation frequencies, as can be understood from the above-described characteristics of disturbance of the disk.
If the servo-loop gain has a value that is greater than necessary, unnecessary noise also is amplified more than necessary, thereby causing current flow in the actuator and unnecessary power consumption. Unnecessary power consumption results in a temperature rise in the actuator and a temperature rise in the overall apparatus. Unnecessary power consumption also causes the actuator to generate noise.
In an MCLV method in which a sample servo optical disk having clock marks or wobble marks, the number per track of which is constant, and which are pre-recorded radially from the center of the disk, is rotated using an MCLV process, since the number of sampling points per track is maintained constant, the sampling frequency becomes lower as the rotation frequency becomes lower, and becomes higher as the rotation frequency becomes higher. Accordingly, if the servo-loop gain is determined so as to suppress disturbance at the highest rotation frequency (at the inner circumference), since the sampling frequency is smaller at an outer circumference having a lower rotation frequency, the servo system becomes unstable or incapable of maintaining control.
On the other hand, if the servo-loop gain is determined by adjusting the sampling frequency at the outer circumference, servo deviation cannot be suppressed so as to remain within an allowable range at the inner circumference.
For example, recent recording/reproducing systems that use a domain wall displacement detection (DWDD) method to achieve higher density recording on optical disks, require greatly reduced servo deviation characteristics. Also, recent trends to reduce the size of recording/reproducing systems require a corresponding reduction in power consumption. Accordingly, it is desirable to provide a servo control circuit having high accuracy (fidelity) and low power consumption.